How to fix AD to AAD synchronization.

Question

Initially, Microsoft accounts were created for school employees and we used them. After that, I created a server to which I successfully synchronized Azure AD accounts with a local AD via Azure AD Connect. A little later, I lost access to the server and I simply reinstalled another server on top, created new AD accounts and decided to synchronize again, but then problems arose. It says that the UPNs are duplicated and errors occur when trying to synchronize. Later, I manually tried to change the ImmutableID so that the synchronization was successful, but it also did not help. It seems that the accounts are still linked to the old server and nothing works, what could be the reason? How can I synchronize in a new way? We exclude the possibility of deleting Azure AD accounts and creating new ones, since there are letters in the mail and many files in OneDrive.